The MRI apparatus is an apparatus which measures a nuclear magnetic resonance (NMR) signal generated by the object, especially, the spin of nuclei which form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding is given to NMR signals by the gradient magnetic field and frequency encoding is also given to the NMR signals, and the NMR signals are measured as time-series data. The measured NMR signals are reconstructed as an image by a two-dimensional or three-dimensional Fourier transform.
Techniques for capturing a blood vessel image of an object without using a contrast medium (hereinafter, referred to as non-contrast MRA techniques) have been put to practical use in the MRI apparatus. For example, there are techniques disclosed in PTL 1 to PTL 4. In the imaging techniques in PTL 1 to PTL 4, the following steps (a) and (b) are executed in an examination scan.
(a) Under electrocardiographic synchronization, echo signals equivalent to the predetermined amount of slice encoding are collected using a high-speed spin echo (hereinafter, referred to as FSE) sequence. The FSE sequence is repeated every plural cardiac beats in a predetermined signal acquisition time (Acquisition Time, AT) after predetermined delay time (Delay Time, DT) from the electrocardiographic synchronization signal.
(a-1) DT and AT are adjusted, and the signal acquisition time of the FSE sequence is set in systole. In addition, a gradient magnetic field pulse (dephase pulse) which causes large phase dispersion in the spin of an artery with high blood flow speed and does not cause large phase dispersion in the spin of a vein with low blood flow speed is applied in a predetermined gradient magnetic field direction. By the application of this dephase pulse, echo signals from the artery with high blood flow speed are suppressed, thereby acquiring a vein image.
(a-2) DT and AT are adjusted, and the signal acquisition time of the FSE sequence is set in diastole. In addition, a gradient magnetic field pulse (rephrase pulse) to compensate for the phase dispersion due to the blood flow speed is applied in a predetermined gradient magnetic field direction. As a result, an arteriovenous image in which an artery image and a vein image are included is acquired.
(b) A difference image is created using the data of the vein image acquired in (a-1) and the arteriovenous image acquired in (a-2). When creating the difference image, weighted differential processing is performed as necessary. For example, the signal strength of a diastolic image is multiplied by a predetermined weighting coefficient, for example, 0.8, and then a difference between the resultant image and the systolic image is calculated. As a result, the vein image is removed from the arteriovenous image, and the artery image can be acquired. A constant set in advance on the basis of experience or the like or the value input by the user is used as a weighting coefficient used for weighting.
Before the above step (a), it is necessary to perform a preparatory scan of the following (c). (c) A preparatory scan is performed before the FSE sequence of (a) using the imaging conditions of lower spatial resolution than the FSE sequence of this measurement of (a), thereby acquiring an image for each cardiac time phase. The user observes an image for each cardiac time phase of the preparatory scan, and selects as a systolic image an image in which only the vein image appears on the highest signal and selects as a diastolic image an image in which both the vein image and the artery image appear on the highest signal. DT and AT are determined such that the above FSE sequence of (a-1) is performed in the cardiac time phase of the selected systolic image. Similarly, DT and AT are determined such that the above FSE sequence of (a-2) is performed in the cardiac time phase of the selected diastolic image.
In addition, the FSE sequence by which a high-quality image is acquired is disclosed in PTL 5, for example.
On the other hand, as a non-contrast MRA technique, a PC (Phase Contrast) method is widely known by PTL 6 to PTL 8 and the like. The PC method is an imaging method of using the phenomenon in which when a bipolar gradient magnetic field that is a pair of gradient magnetic field pulses with the opposite polarities and the same magnitude is applied, a phase change according to the blood flow speed occurs in the spin in tissue with flow speed, such as a blood flow, while no phase change occurs in stationary tissue. By performing imaging while applying a bipolar gradient magnetic field in a predetermined direction, a change in the phase of a blood flow portion is obtained as an image. By inverting the polarity of the bipolar gradient magnetic field pulse, two images are obtained eventually. By calculating the difference to remove a signal of the stationary portion, an image of only the blood flow portion can be acquired. The pixel value of the image corresponds to the flow speed of the blood flow.